In systems which are controlled by a motor, it is desirable to accurately sense the current that is being driven through the motor. Typically, the current along with several other parameters is monitored and input to a feedback control loop which refines the operation of the motor. For example, in a DC motor, the current, shaft position, speed and direction of the rotating motor may all be measured, and those measurements input to a motor control circuit, which forms control inputs to the driver to adjust the operation of the motor.
In small, lightweight devices, it is desirable for the footprint of the current sensor to be as small as possible, and the weight of the current sensor to be minimised. It is also desirable for the current sensor to sense current without dissipating any of that current. A Hall sensor is a magnetic current sensor which is activated by an external magnetic field acting on it. It is thus able to provide a non-intrusive measurement of the current being driven through a motor by detecting the magnetic field generated by the current flow. It is also available packaged into a small IC. The Hall sensor produces an output voltage which varies as a function of the magnetic field density around it. Hence, by measuring the output voltage of a Hall sensor located in close proximity to the path of the driver output, the driving current of the motor is determined.
However, Hall sensors which are available packaged into a small IC suffer from the problem that their operation is very sensitive to their thermal surroundings. They are very accurate at measuring current in a narrow temperature window. However, outside of that narrow temperature window, their accuracy drops. As a motor is driven, the circuit boards on which the motor and the Hall sensor are located increase in temperature to levels outside of the narrow temperature window at which the Hall sensor provides an accurate current measurement. A typical circuit board on which the motor is driven can be expected to vary in temperature during operation from 15° C. to 60° C.
Large Hall sensors are available that are more temperature stable than the small Hall sensors discussed above, however these are not suitable for small, lightweight applications.
It would be possible to fully characterise the temperature characteristics of the circuit board and the Hall sensor during manufacture. Then the Hall sensor readings could be dynamically adjusted during operation to account for the current temperature of the circuit board. However, this would increase the time, complexity and cost of manufacturing the motor circuit.